1. Technical Field of the Invention
The embodiments of the invention relate to wireless communications and more particularly to sampling frequency offset estimation and adjustment in a receiver of a multiple-input and multiple-output system.
2. Description of Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems, the Internet and to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera, communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via a public switch telephone network, via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it typically includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). The receiver may be coupled to an antenna and the receiver may include a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies them. The one or more intermediate frequency stages mix the amplified RF signals with one or more local oscillators to convert the amplified RF signal into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
The transmitter typically includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier stage. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillators to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
In traditional wireless systems, the transmitter may include one antenna for transmitting the RF signals, which are received by a single antenna, or multiple antennas, of a receiver. When the receiver includes two or more antennas, the receiver generally selects one of them to receive the incoming RF signals. In this instance, the wireless communication between the transmitter and receiver is a single-output-single-input (SISO) communication, even if the receiver includes multiple antennas that are used as diversity antennas (i.e., selecting one of them to receive the incoming RF signals). For SISO wireless communications, a transceiver includes one transmitter and one receiver. Currently, most wireless local area networks (WLAN) that are IEEE 802.11, 802.11a, 802,11b, or 802.11g employ SISO wireless communications.
Other types of wireless communications include single-input-multiple-output (SIMO), multiple-input-single-output (MISO), and more recently, multiple-input-multiple-output (MIMO). In a SIMO wireless communication, a single transmitter processes data into radio frequency signals that are transmitted to a receiver. The receiver includes two or more antennas and two or more receiver paths. Each of the antennas receives the RF signals and provides them to a corresponding receiver path (e.g., LNA, down conversion module, filters, and ADCs). Each of the receiver paths processes the received RF signals to produce digital signals, which are combined and then processed to recapture the transmitted data.
For a multiple-input-single-output (MISO) wireless communication, the transmitter includes two or more transmission paths (e.g., digital to analog converter, filters, up-conversion module, and a power amplifier) that each converts a corresponding portion of baseband signals into RF signals, which are transmitted via corresponding antennas to a receiver. The receiver includes a single receiver path that receives the multiple RF signals from the transmitter.
For a multiple-input-multiple-output (MIMO) wireless communication, the transmitter and receiver each include multiple paths. In such a communication, the transmitter parallel processes data using a spatial and time encoding function to produce two or more streams of data. The transmitter includes multiple transmission paths to convert each stream of data into multiple RF signals. The receiver receives the multiple RF signals via multiple receiver paths that recapture the streams of data utilizing a spatial and time decoding function. The captured receive signals are jointly processed to recover the original data.
With the various types of wireless communications (e.g., SISO, MISO, SIMO, and MIMO) and standards (e.g., IEEE 802.11, IEEE 802.11a, IEEE 802.11b, IEEE 802.11g, IEEE 802.11n, extensions and modifications thereof), a large number of combination of types and standards is possible. However, when a wireless communication utilizes MIMO format for communicating between a receiver and a transmitter, complexities result due to the multiple transmission and receive paths for a given signal. Estimating channels at the receiver for a received signal generally requires taking into account the multiple signal paths from the transmitter.
A number of factors are known to impair or degrade the received signal when the receiver is performing channel estimation. One of these impairments is carrier frequency offset (CFO). Another is sampling frequency offset (SFO). Carrier frequency offset occurs when the carrier frequency (fc) in the receiver is not at the exact frequency of the transmitted carrier. Similarly, sampling frequency offset occurs when the actual sampling frequency (fs) of the received signal is not at the exact desired sampling frequency expected at the receiver to sample the incoming signal.
In a typical SISO system, CFO and SFO estimation and adjustment are fairly simple to implement. However, in MIMO systems, CFO and SFO estimation and adjustment are more complicated to implement due to the use of multiple antennas for transmission and reception. Furthermore, when MIMO systems utilize a technique that transmits a particular data stream during different time blocks, the complexities of estimating and adjusting for CFO and SFO are compounded by additional offsets that result from timing separation of the transmitted data.
Channel estimation is a technique employed in the receiver to obtain an estimation of the transmission path of the transmitted signal, in which the estimation is used to recover the signal. With the use of multiple antennas in a MIMO system, channel estimation generally involves matrix operations, instead of a simple vector for SISO system. The CFO and/or SFO experienced in the receiver may introduce phase errors, resulting in undesirable placement of signal points in a signal constellation. Generally, the phase error causes the received signal points to rotate within the constellation, so that signal points are not disposed at corrects locations within the constellation. Since channel estimation involves interpreting the received signals for correct transformation to process the received signal, CFO and SFO induced phase errors may make channel estimation difficult or not possible to achieve.
Accordingly, there is a need to provide a channel estimation technique that considers presence of CFO and/or SFO in a MIMO receiver.